Dane.Kouttron

[10.2.09]
Quick
Modification to a Tap Light

What? There's a dark stair
well i traverse every night to get to
my apartment. I modified a cheap tap-light to be [Tap-> stay on
for
40 seconds, then shut off]. The mod takes all of 10 minutes to do, and
is really useful if you don't want to go back and shut it off.

This
is also
useful in like a closet, where you hit the button look for things, and
it shuts off by itself.

Hardware:

Component

Purpose

Image

N
channel
Mosfet

For
those who are just starting EE things, the mosfet is basically a switch.
image from:
http://www.geocities.com/radio107mhz/mosfet1.gif

Momentary Switch

Replacement
for the
switch provided on the unit. Is connected when pushed and otherwise
open circuit.
from http://alexplorer.net/guitar/mods/momentary.jpg

Capacitor

Timing
capacitor.
Based on Capacitance & Resistance, you determine on time
constant
(or amount of time the system stays on)

Resistor (timing)

Timing
Resistor, Based
on C&R you determine on time

9v
battery & clip

I ran
it on a 9v, because i have a
bunch of em

White Led's

way
more energy efficient and brighter
than the incandescent it comes with. (image from digikey)

Build
&Test:

Action

Purpose

Image

Basic Schematic

A momentary switch, mosfet, led's
resistor and cap. Time constant approximated by how long it would take
for the bleed resistor to reduce the starting voltage (9v) to the
dropout voltage of the gate (depends on the mosfet). If your working
with unlabeled parts, test values until you find the time constant you
want, as gate capacitance is probably unknown.

momentary switches are on only while your holding them down, like when
you smack the taplight

Soldering
it
together, boardless style

Who
says
you need
perfboard :)

Top View

Here,
you can
see the
momentary switch, and the newly
installed led's Yes, there are only 2, they were scavenged from
something. Yes, you can use more than 2.

Everything shoved into the
back of the tap-light

I
shoved
everything into
the back of the tap-light. The
system could have used 4AA's like it was designed to, however i was in
a rush and decided i'd rather run it on some 9v's.

Button it up

Align
the
springs, check
that the button fits properly and
button the whole thing up.

Notes:Analog
circuit
design has been relatively abandoned in comparison to digital design.
Sure, its more complex to get working, and sometimes finicky in terms
of noise, but it excels in other areas, like filtering and other
simpler designs. In the simple cases, like above, its wonderful. When i
first contemplated this idea, i was thinking, 'well i would have to get
a 555, or an attiny...', then i realized that was silly and a waste of
good hardware. The general idea here is simplicity can also be elegant.
Oh, you also get that nice analog fade as it shuts off. :)

What
is Actually Going On

1.
Everything is in the off state:

The modified taplight is off, just sitting there minding its own
business. No significant currents happen anywhere.

2. You
Smack The Taplight
The voltage of the battery suddenly appears at the gate of the mosfet,
turning on the led's. This also fills the Timing capacitor rather
quickly with charge.

3. The
you climb up the stairs, after releasing the tap light.

The charge that was stored in the cap slowly depletes across the timing
resistor.
As the cap voltage drops, the mosfet remains on, until the voltage
approaches the fet's threshold, wherein it begins to reduce the current
flow (fet now operates in the analog region)

Other note:
This site is intended to explain things so others can get a grasp at
electronics.
While this particular project may not be helpful to you, it might be
helpful to someone younger / less experienced than you. (middle
schoolers)